First, there's a clause in most contracts it's good to keep in mind, that failure to enforce something isn't a guarantee it won't be enforced in the future. Second, when you want to get prey to be in position to easily take them, hunters often use delicious bait. Finally, while the extant relatively sovereign polities exist and some competition is undertaken between them, many despicable means are employed by criminal spooks. These criminal organizations have uses for decentralized communications, cryptography, and all manner of privacy enhancing tools, such as Tor, and all of these things are products of government research. When it's time to end these tools' availability to the public, that availability will end. Little is easier than censoring data, and nothing harder needs to be done to end it.

Yes, it is the same physical infrastructure that transmits data on the dark web that transmits data on the clear net.